The freshwater pearl mussel Margaritera margaritifera is widely distributed in the northern hemisphere from Eastern North America across Europe to Siberia and Japan. It is estimated that the freshwater pearl mussel is now present in 5% of its former range (compared with the estimates of known localities at the beginning of the century).

The life cycle of the pearl mussel is complicated by the presence of the parasitic glochidial stage. Survivorship of this stage depends on locating a suitable host and the subsequent settlement of the post glochidial stage in a suitable habitat. The mortality of these stages is very high especially in the fast flowing habitats typical for the pearl mussel. To vercome this problem the life span is great (to 130 years) and the fecundity high. Mussels in large populations tend to be dioecius but in low density conditions can become self fertile hermaphrodites. Theoretically therefore small populations of adults could be viable. The period of viability however would depend on the life span of these adults and the survivorship of the offspring. Should recruitment cease any population will survive only through the remainder of the current individuals life span which could be as long as 100 years.As with many rivers in Britain and Europe there is no evidence of recruitment in that no juvenile or small mussels can be found. Given the size of the mussels found during this survey we would expect these mussels to be nearing the end of their lifespan. Consequently without recruitment we would expect that there would be few if any mussels remaining within 10-20 years. Results from rivers surveyed in Austria, Germany, Ireland, Scotland, Norway indicate that there is no recruitment in most rivers and there has not been any for many years then we believe that these populations are in terminal decline.

There has been a dramatic decline in Margaritifera populations throughout much of its range with eutrophication of rivers, intensification of agriculture, land drainage and afforestation and continuing pearl fishing implicated as the major attributing factors.